As shown in FIG. 4, an image-inputting apparatus, which is used in facsimiles, digital copying machines, scanners and other apparatuses, is provided with a line sensor 51 for reading an original document 50. The line sensor 51 has a construction wherein light-receiving elements are aligned in a straight-line shape.
Light, reflected from the original document 50, is focused on the light-receiving elements in the line sensor 51 by a lens 49, and analog signals, which have respective levels corresponding to the quantities of light received by the light-receiving elements, are released from the line sensor 51 as a serial signal. Here, it is possible to read the entire portion of the original document 50 by shifting the line sensor 51 or the original document 50.
The analog signals from the line sensor 51 are converted into digital signals by a signal-processing circuit. FIG. 5 shows one example of the signal-processing circuit wherein CCDs (Charge Coupled Devices) are used as the light-receiving elements of the line sensor 51.
The signal-processing circuit is provided with an amplification section 52 for amplifying such a minute output from the line sensor 51 to an appropriate level and an A/D (analog/digital) converter 53 for converting a signal from the amplification section 52 into a digital signal.
A lower-limit reference voltage (Vref-), which is used in the A/D converter 53, is obtained by converting data from a black-correction-value storing section 54 into analog signals by using a D/A (digital/analog) converter 55, and an upper-limit reference voltage (Vref+), which is also used in the A/D converter 53, is obtained by converting data from a white-correction-value storing section 56 into analog signals by using a D/A (digital/analog) converter 57.
Here, the black-correction-value storing section 54 stores output voltage values (output voltages in the dark state) from the CCDs as its data that are obtained when no light illuminates the line sensor 51, and the white-correction-value storing section 56 stores output voltage values (output voltages in the bright state) from the CCDs as its data that are obtained when light reflected from a white plate illuminates the line sensor 51.
As one example of the A/D converter 53, FIG. 6 shows a basic construction of a flash-type A/D converter which is capable of providing high-speed conversion that is suitable for the line sensor 51 consisting of CCDs.
In the flash-type A/D converter, voltages, which are obtained by dividing the potential difference between the reference voltages Vref+ and Vref- by using resistors 61, are compared with input signals (Vin) by using comparators 62. Then, an encoder 64 processes the outputs from the comparators 62 by means of AND circuits 63, thereby providing digital outputs.
In the above-mentioned conventional arrangement, however, it is necessary to install an A/D converter 53 with high resolution in order to read an original document 50 such as a photograph with high accuracy. Consequently, the problem is that the cost of the signal-processing circuit tends to become expensive.
Moreover, as the resolution of the A/D converter 53 is improved, the input capacity increases because of an increased degree of integration; this makes it difficult to operate the A/D converter 53 at high frequencies. Furthermore, it is necessary to provide a trimming process in order to increase accuracy of resistances of the resistors 61 that divide the potential difference between the reference values Vref+ and Vref-. For this reason, it is actually very difficult to realize a resolution of more than 10 bits.
In order to solve these problems, Japanese Laid-Open Patent Application No. 108867/1991 (Tokukaihei 3-108867) discloses an image-reading apparatus wherein signals, accumulated in the image sensor, are read out a plurality of times in a divided manner. In other words, a plurality of divided areas are provided between the white-correction value and the black-correction value; the upper limit and the lower limit of each area are defined as Vref+ and Vref- respectively; and an analog-to-digital conversion is carried out for each area. With this arrangement, it is possible to realize an analog-to-digital conversion with high resolution even in the case of using A/D converters having inferior resolution.
In this apparatus, however, for example, when the analog-to-digital conversion having a resolution of 8 bits is carried out by using A/D converters having a resolution of 6 bits, the signals accumulated in the image sensor have to be read out four times. This causes a new problem in that time required for inputting images is lengthened.